The present invention pertains generally to optical communications, and more particularly to a micro-mechanical optical modulator.
FIG. 1 depicts passive optical network 100. Network 100 includes central office or head-end terminal 102, splitter 110, wavelength routing device 112 and a plurality of network units 114i, i=1xe2x88x92n, interrelated as shown.
Central office 102 includes transmitter 104 and receiver 118. Transmitter 104 incorporates active optical source 106, such as a multi-frequency laser or light-emitting diode. Transmitter 104 generates optical signal 108, which is a wavelength division multiplexed (xe2x80x9cWDMxe2x80x9d) signal. WDM signals comprise multiple independent data channels, each of which is assigned to a distinct optical wavelength. Central office 102 sends information over WDM optical signal 108 to the plurality of network units 1141-114n, which each receive information over one of the distinct wavelengths.
Wavelength routing device 112 de-multiplexes WDM signal 108 into its constituent spectral components (optical wavelengths) 108i, i=1xe2x88x92n, such that the spectral components 108i are spatially separated from one another. Each spatially-separated spectral component 108i is then routed, over waveguides 113i, i=1xe2x88x92n, to the appropriate network unit 114i as a function of wavelength. In some embodiments, the waveguides are optical fibers.
With reference to FIG. 2, waveguide 109i delivers spectral component 108i to splitter 220 in network unit 114i. Splitter 220 routes a first portion 222 of the power of spectral component 108i to receiver 226, and a second portion 224 to transmitter 228.
Information is advantageously sent in packets to network unit 114i via spectral component 108i. The packets contain information (ie., television programming, incoming e-mail, etc.) for processing as well as continuous wave (xe2x80x9cCWxe2x80x9d) light or xe2x80x9coptical chalkboardxe2x80x9d which can be modulated with information. First portion 222 of spectral component 108i is converted to an electrical signal that is representative, in part, of the information contained in the packet. The electrical signal is then routed to processing electronics, not shown. Optical modulator 230 in transmitter 228 modulates information on the CW light that is contained in second portion 224, generating modulated (ie., information-carrying) spectral component 116i. The information modulated onto the CW light can be, for example, phone message 232 or information 234 destined for the Internet.
Modulated spectral components 116i, i=1xe2x88x92n, returned from network units 1141-114n, are multiplexed by wavelength routing device 112 into WDM signal 116. Splitter 110 routes signal 116 to receiver 118 in central office 102.
Optical modulator 230 that is used in network 100 can be a micro-mechanical optical modulator. This type of modulator typically uses optical interference principles to vary the signal strength of an optical signal (e.g., a carrier signal, such as the CW light of second portion 224 of spectral component 108i). One well-known implementation of such a modulator is depicted in FIG.3.
Modulator 230 depicted in FIG. 3 incorporates a movable mirror, realized as movable layer or membrane 344 that is supported by supports 346 above fixed multi-layer mirror 342. The fixed multi-layer mirror is disposed on substrate 340. Membrane 344 forms a Fabry-Perot cavity, well known in the art, with underlying fixed mirror 342. Membrane 344 and fixed mirror 342 are electrically connected to controlled voltage source 350.
In operation, controlled voltage source 350 applies a voltage across membrane 344 and fixed mirror 342 thereby generating an electrostatic force. This force draws membrane 344 toward fixed mirror 342 along vector 452, as depicted in FIG. 4. When the applied voltage is withdrawn, membrane 344 returns to the quiescent or unactuated position depicted in FIG. 3.
As membrane 344 moves toward fixed mirror 342, the size of the Fabry-Perot cavity (i.e., the size of gap 448 between membrane 344 and fixed mirror 342) changes. This change is accompanied by a change in the reflectivity of modulator 230. The optical interference principle that governs this behavior is described with reference to FIG. 5.
In a typical prior art modulator, membrane 344 has an optical thickness that is an odd integer multiple of one-quarter of the operating wavelength (xe2x80x9cxcex/4xe2x80x9d) of the modulator. Fixed multi-layer mirror 342 consists of anti-reflection layer 554 and coating layer 556 that each have an optical thickness that is an odd integer multiple of xcex/4. Membrane 344 and coating layer 556 have a refractive index that is equal to the refractive index of substrate 340. Anti-reflection layer 554 has a refractive index that is about equal to the square root of the refractive index of the substrate 340. See, Marxer et al., xe2x80x9cMHz Opto-Mechanical Modulator,xe2x80x9d Transducers ""95xe2x80x94Eurosensors IX, The 8th International Conference on Solid-State Sensors and Actuators, and Eurosensors IX, Stockholm, Sweden, Jun. 25-29, 1995, pp. 289-292.
In a modulator that is configured as described above, modulator reflectivity is at a high value (i.e., a relative maxima) when the size of gap 448 is an odd integer multiple of xcex/4. This configuration generates a constructive interference condition since the round trip distance of the optical signal from membrane 344, across gap 448, over coating layer 556 and back again is an integer multiple of xcex. That is, the optical signal is in-phase.
Conversely, modulator reflectivity is reduced to zero (i.e., a relative minima) when the size of gap 448 is zero or an even integer multiple of xcex/4. This configuration generates a destructive interference condition since the round trip distance of the optical signal is an integer multiple of 3xcex/2-180 degrees out of phase.
FIGS. 6 and 7 depict the performance of micro-mechanical modulator 230 having the layer arrangement and layer characteristics shown in FIG. 5 and that is designed for a wavelength, xcex, of 1570 nanometers.
FIG. 6 depicts reflectivity as a function of wavelength for modulator 230 wherein membrane 344 is silicon and a substrate 340 is silicon. Plot 658 shows the maximum reflectivity condition wherein the size of gap 448 is an odd integer multiple of xcex/4xe2x80x94in this case, 3xcex/4. For this particular configuration, maximum reflectivity is shown to be about 97 percent. Plot 660 shows the minimum reflectivity condition wherein the size of gap 448 is an even integer multiple of xcex/4xe2x80x94in this case, 2xcex/4. For this particular configuration, minimum reflectivity is zero at the design wavelength of 1570 nanometers.
FIG. 7 depicts reflectivity as a function of wavelength for modulator 230 wherein membrane 344 is silicon and substrate 340 is germanium. Plot 762 shows the maximum reflectivity condition wherein the size of gap 448 is an odd integer multiple of xcex/4, which, again, is 3xcex/4. For this configuration, maximum reflectivity is, as before, about 97 percent. Plot 764 shows the minimum reflectivity condition wherein the size of gap 448 is an even integer multiple of xcex/4, here, 2xcex/4. Minimum reflectivity is zero at the design wavelength of 1570 nanometers.
FIGS. 6 and 7 demonstrate that silicon and germanium can be used interchangeably as the substrate with substantially no impact on modulator performance. FIGS. 6 and 7 also illustrate a shortcoming of this particular modulator arrangement. Specifically, while insertion loss is minimized for the modulator configuration described above, the operating bandwidth is relatively narrow. That is, minimum reflectivity rises relatively rapidly with deviations from the design wavelength (e.g., 1570 nanometers) so that contrast (ie., the ratio of the maximum reflectivity to the minimum reflectivity) rapidly decreases. Consequently, the art would benefit from an improved modulator design possessing a greater operating bandwidth, or at least the ability to trade insertion loss for bandwidth, as desired.
A micro-mechanical Fabry-Perot cavity optical modulator in accordance with the present invention provides the ability to trade insertion loss for bandwidth.
Optical modulators described in this specification include a substrate having a multi-layer mirror disposed thereon. The multi-layer mirror comprises an anti-reflection layer that is disposed on the substrate, and a coating layer that is disposed on the anti-reflection layer. The optical modulator also has a membrane that is separated by a gap from the coating layer. The membrane is movable, and, as it moves, the size of the gap changes. The reflectivity of the modulator is dependent upon the size of the gap. Specifically, modulator reflectivity is at a maximum when the size of the gap is an odd integer multiple of one-quarter of the operating wavelength of the modulator and at a minimum when the size of the gap is zero or even integer multiples of one-quarter of the operating wavelength.
In accordance with the illustrative embodiment of the present invention, the membrane and the coating layer have the same refractive index. Furthermore, the anti-reflection layer has a refractive index that is equal to the square root of the refractive index of the substrate. Additionally, the combined thickness of the membrane and the coating layer is equal to an integer multiple of one-half of the operating wavelength of the modulator. This thickness restriction is different than prior art Fabry-Perot cavity modulators, which typically independently restrict membrane thickness and coating layer thickness, each to multiples of one quarter of the operating wavelength.
By relaxing the requirements imposed by the prior art on layer thickness, modulator performance parameters can be optimized. Specifically, optical bandwidth can be traded for insertion loss and vice versa, as suits the specifics of a particular application.